This invention relates to a new thread control mechanism for a sewing machine. The prior art thread control mechanisms, do not always apply an even tension to the needle threads and as a result uniform needle thread loops are not always produced on the underside of the work and the finished product often does not have uniform stitches. Also, portions of the thread control mechanism, that must be adjusted for the particular type of thread being used, are not conveniently accessible. The prior art thread control mechanisms are difficult and time consuming to rethread in the event of thread breakage or to change from one type of thread to another for example from cotton to polyester thread. Improperly adjusted thread control mechanisms result in thread breakage, non-uniform stitches and non-uniform needle loops. Considerable time, and thus cost, is required even by skilled sewing machine operators to obtain proper adjustment of the thread control mechanism and to rethread the machine or to change threads.
U.S. Pat. No 3,368,508 discloses a needle thread take-up device for a cover stitch sewing machine. The invention disclosed in this patent includes an adjustable eyelet member that is fixed to the sewing machine housing and a take up finger that is connected to the needle bar's oscillating lever and extends through a slot formed in the sewing machine housing. The needle thread extends through the fixed eyelet member and then through the eyelet member of the take up finger. Although the arcuate path followed by the take up finger is generally vertical, since it moves along an arc, its movement includes horizontal components and its vertical component is less than the vertical component of the needle bar. The take up device of this patent also includes a U-shaped thread guide, carried by the top of the needle bar, having aligned thread eyelets formed in the parallel legs of the U-shaped thread guide. The U-shaped thread guide includes an adjustable spring arm projecting between the legs of the U-shaped member that is biased into engagement with the needle thread. The U-shaped thread guide and adjustable spring arm cooperate with a take up that is fixed to the sewing machine frame and this combination functions to regulate the size of the needle thread loop. The invention of this patent resides in the cooperative arrangement of the fixed take up with the movable U-shaped thread guide and the adjustable spring arm. The precise adjustment of the various components of this complex needle thread take-up device is critical to proper operation and it is difficult and time consuming to change threads. Furthermore, the vertical component of movement of the take up finger is less than the vertical component of movement of the U-shaped thread guide, and as a result the movement of the U-shaped thread guide and the take up finger are not synchronized. This non-synchronized movement creates tension in the thread that extends between these components, which results in non uniform needle thread loops, non uniform stitches and thread breakage.